Electrolytes for calcium-based secondary cell and calcium-based secondary cell comprising the same

ABSTRACT

The present disclosure concerns an electrolyte suitable for calcium-based secondary cells, comprising calcium ions and an electrolyte medium, wherein the electrolyte is not solid at standard conditions and wherein the electrolyte medium includes at least two distinct non-aqueous solvents.

The present disclosure discloses electrolytes particularly suitable foruse in a calcium-based secondary cell, a calcium-based secondary cellwhich can be effectively operated under mild temperature and voltageconditions, a non-aqueous secondary battery containing the cell as wellas a vehicle, an electronic device or a stationary power generatingdevice containing the battery.

BACKGROUND

Secondary (i.e. rechargeable) electrochemical cells and batteries are apower source widely used in information-related devices, communicationdevices (such as personal computers, camcorders and cellular phones) aswell as in the automobile industry or in stationary power generatingdevices. Conventional lithium-based cells typically include a positiveelectrode (also referred to as “cathode”) and a negative electrode (alsoreferred to as “anode”) whose active materials are capable of acceptingand releasing lithium ions, as well as an electrolyte arranged betweenthe electrodes and including lithium ions. Lifetime of conventionallithium-based secondary cells and batteries is not always satisfactory.The formation of a solid-electrolyte interphase (SEI) on the negativeelectrode seems to be a key phenomenon when using organic solvents inrechargeable batteries. Notably, the solvent seems to decompose oninitial charging and form a solid layer called the solid electrolyteinterphase (SEI) which is electrically insulating but should exhibitionic conductivity in order to allow successful battery operation. Theinterphase is thought to prevent further decomposition of theelectrolyte upon battery cycling. SEI thus seems to play a key role incontrolling cell electrochemical process, delaying of capacity fade,setting cycle life and ultimately determining cell performances. Forthis reason it would be desirable to provide a secondary cell/batteryendowed with of a good quality SEI and/or operate the same to minimizethe impact that SEI with unsatisfactory quality may have on cell/batteryperformances. Energy storage properties are further aspects that are notalways satisfactory in conventional lithium-based secondary cells andbatteries. Therefore, it would be desirable to have secondary cellsendowed with higher energy density and higher capacity of its negativeelectrode. At the same time, the cell should be rechargeable whenoperated under mild temperature conditions and potential windows so asto be a commercially viable energy storage device compatible with a vastpanel of final applications.

The present disclosure discloses electrolytes particularly suitable foruse in a calcium-based secondary cell, a calcium-based secondary cellwhich can be effectively operated under advantageous temperature andvoltage conditions, a non-aqueous secondary battery containing the cellas well as a vehicle, an electronic device or a stationary powergenerating device containing the battery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a calcium-based secondary cell;

FIG. 2 are diagrams reporting cyclic voltamperommetry measurementsproving the electrodeposition and stripping of calcium electrodes incertain experimental conditions. Tests conditions are 0.3M Ca(ClO₄)₂ inEC_(0.5):PC_(0.5); a) at 1 mV/s and various temperatures; b) at 50° C.and various sweep rates. Current collector is stainless steel;

FIG. 3 is a picture of a stainless steel after calcium electroplatingmode at −1 V vs. Ca²⁺/Ca in 0.3M Ca(BF₄)₂ in EC_(0.5):PC_(0.5) at 75°C.;

FIG. 4 are three scanning electron microscope images of copper electrodea) before and b), c) after calcium deposition in 0.3M Ca(BF₄)₂ inEC_(0.45):PC_(0.45):DMC_(0.1). Calcium deposits were achieved at 75° C.in potentiostatic mode at b) −1 V vs. Ca²⁺/Ca during 80 h and c) −1.5 Vvs. Ca²⁺/Ca during 200 h;

FIG. 5 is a diagram reporting the EDS quantitative analysis on calciumdeposited in experiment c) as defined in FIG. 4;

FIG. 6 is a diagram reporting the synchrotron X-ray diffraction patternof the deposit formed on a copper disk and stainless steel plunger aftercalcium deposition in 0.3M Ca(BF₄)₂ in EC_(0.45):PC_(0.45):DMC_(0.1)achieved in potentiostatic mode at −1.5 V vs. Ca²⁺/Ca during 200 h;

FIG. 7 is a diagram reporting cyclic voltamperommetry measurements onelectrodeposited calcium obtained after 200 h potentiostatic depositionat −1.5 V vs. Ca²⁺/Ca, the electrolyte being 0.3M Ca(BF₄)₂ inEC_(0.45):PC_(0.45):DMC_(0.1) using various current collectors (calcium,aluminum, gold). In all cases, the sweep rate was 1 mV/s;

FIG. 8 is a diagram reporting cyclic voltamperommetry measurements onelectrodeposited calcium obtained after 200 h potentiostatic depositionat −1. 5 V vs. Ca²⁺/Ca, the electrolyte being Ca(BF₄)₂ inEC_(0.45):PC_(0.45):DMC_(0.1) at various salt concentrations (0.3, 0.65and 1 M). In all cases, the sweep rate was 1 mV/s and the currentcollector was gold;

FIG. 9 is a diagram reporting PITT measurements of composite filmnegative electrode containing tin electrodes cycled in 0.3M Ca(BF₄)₂ inEC_(0.45):PC_(0.45):DMC_(0.1) at 75° C.;

FIG. 10 are two pictures representing scanning electron microscopyimages of composite powder electrode (composition: tin 80% wt. andconductive carbon black 20% wt.—conductive carbon black was Carbon SuperP®, also referred to as “C_(sp)” commercially available from TIMCAL) a)before and b) after a complete reduction (PITT measurements). Cyclingwas performed at 75° C. using 0.3M Ca(BF₄)₂ inEC_(0.45):PC_(0.45):DMC_(0.1) electrolyte;

FIG. 11 is a diagram reporting the particle size distribution of tinparticles used in experiments a) and b) as per FIG. 10;

FIG. 12 is a diagram reporting the PITT measurements of composite filmnegative electrode containing silicon cycled in 0.3M Ca(BF₄)₂ inEC_(0.45):PC_(0.45):DMC_(0.1) at 75° C.;

FIG. 13 are diagrams reporting cyclic voltamperommetry measurementsproving the electrodeposition and stripping of calcium electrodes incertain experimental conditions. Tests conditions are 0.3M Ca(BF₄)₂ inEC_(0.5):PC_(0.5) at 100° C. Current collector is stainless steel.Cycles 1, 2, 3 and 10 are reported on FIGS. 13 a, b, c and d,respectively.

DISCLOSURE

In one aspect, the present disclosure discloses an electrolytecomprising, such as consisting of calcium ions and an electrolytemedium, wherein the electrolyte is not solid at standard conditions andwherein the electrolyte medium includes at least two distinctnon-aqueous solvents.

The electrolyte is not solid at a temperature of about 20° C. and apressure of 1 atm (hereinafter also referred to as standard conditions).Unless otherwise indicated, “not solid” means that at standardconditions, the electrolyte may be a liquid, a viscous mass or a gel.For example it has a viscosity of lower than 10 cP, such as lower than 8cP, for example lower than 5 cP, when measured using a rheometer (forexample a RheoStress RS600 Rheometer available from HAAKE) at standardconditions.

Advantageously, the electrolyte presently disclosed is endowed with oneor more, such as all of the following properties:

-   -   high ionic conductivity, such as higher than 3 mS/cm, when        measured at standard conditions using a conductivity meter (for        example a Oakton CON 11 standard conductivity meter),    -   allow a good wettability of the battery components (such as        separator and electrodes).    -   substantially free of water, such that for example the        electrolyte contains less than 300 ppm, such as less than 200        ppm, or less than 100 ppm, for example less than 50 ppm of        water, the water content being as measured with the Karl Fischer        titration technique,    -   large electrochemical stability window, such as higher than        4.5V, when measured at about 75° C. using cyclic        voltamperometry. Electrochemical stability window (ESW) is given        in Volt. No indication of reference electrode is necessary since        this ESW is the difference between the stability upon reduction        and upon oxidation and thus would be the same whatever the        reference electrode used. The ESW may be measured at 75° C. by        assembling three electrode tight cells with calcium metal as the        reference and counter electrodes and using a universally        recognized method: the cyclic voltamperometry at a sweep rate of        1 mV/s,    -   thermal stability when measured at a temperature comprised        between about −70° C. and 300° C., such as between about −70° C.        and 270° C. or between about −70° C. and 240° C. or between        −30° C. and 150° C. Thermal stability may be measured with DSC        measurements performed using for example a DSC 204F1 Netzsch        calorimeter, with a heating rate of 10° C./min. Electrolyte        stability is essentially determined by the stability of the        solvents contained therein. A solvent is considered to be        thermally stable at a given temperature when it does not        decompose at that temperature.

Calcium ions may be in the form of a calcium salt, for example andinorganic calcium salt and/or an organic calcium salt. Preferably, thesalt is anhydrous.

The salt may be selected from the group consisting of calciumtetrafluoroborate (Ca(BF₄)₂), calcium perchlorate (Ca(ClO₄)₂) calciumhexafluorophosphate (Ca(PF₆)₂), Ca(CF₃SO₃)₂ and mixtures thereof.(Ca(BF₄)₂) and mixtures thereof may be preferred.

The salt may be dissolved in the electrolyte medium. The salt may bepresent in an amount comprised between 0.05M and 2M, such as between0.1M and 1M, with respect to the volume of the electrolyte.

The electrolyte may be substantially free of other metal ions of Group Iand II of the period table—for example lithium ions, sodium ions,potassium ions. This means that the amount of metal ions other thancalcium possibly presents in the electrolyte is electrochemicallyineffective.

Each solvent present in the medium is substantially free of water.Unless otherwise indicated, substantially free of water means that thesolvent may include water in an amount equal to or lower than 300 ppm,such as equal to or lower than 50 ppm, as measured with the Karl Fischertitration technique.

Advantageously, each solvent present in the medium and/or thecombination thereof is stable at a temperature comprised between atleast −30 and 150° C. (stability window).

Each solvent present in the medium may independently be selected fromthe group consisting of cyclic carbonates, linear carbonates, cyclicesters, cyclic ethers, linear ethers and mixtures thereof.

Cyclic carbonates may be selected from the group consisting of ethylenecarbonate (EC), propylene carbonate (PC), butylene carbonate, vinylenecarbonate, fluoroethylenecarbonate (FEC) and mixtures thereof.

Linear carbonates may be selected from the group consisting of dimethylcarbonate (DMC), diethylcarbonate (DEC), ethyl methyl carbonate (EMC),and mixtures thereof.

Cyclic ester carbonates may be γ-buryrolactone and/or γ-valerolactone.

Cyclic ethers may be tetrahydrofuran (THF) and/or2-methyltetrahydrofuran.

Linear ethers may be selected from the group consisting ofdimethoxyethane (DME), ethylene glycol dimethyl ether, triethyleneglycol dimethyl ether (TEDGE), tetraethyleneglycol dimethyl ether(TEDGE), and mixtures thereof.

In addition or in alternative, the solvent may include dimethylsulfoxide(DMSO) or nitrile solvents (such as acetonitrile, propionitrile, and3-methoxypropionitrile).

Preferably one of the at least two solvents is ethylene carbonate (EC).For example, the electrolyte medium may include ethylene carbonate (EC)and propylene carbonate (PC), such as a combination of formulaEC_(h):PC_(1-h) wherein the ratio is expressed as volume:volume and h is0≦h≦1, such as 0.2≦h≦0.8 or h is 0.5. Mixtures of ethylene carbonate(EC) and propylene carbonate (PC) may be stable between −90° C. and 240°C. The solvent may be for example a combination of ethylene carbonate(EC), propylene carbonate (PC) and dimethyl carbonate (DMC), such as acombination having formula EC_(x):PC_(y):DMC_(z) wherein the ratio isexpressed as volume:volume and 0≦x,y,z≦1 and x+y+z=1.

The at least two solvents may be present in a total (i.e. combined)amount comprised between about 50 and 99% by mass, with respect to 100%by mass of the electrolyte. For example, the solvents may be present inan amount comprised between about 70 and 99% by mass, with respect to100% by mass of the electrolyte. This range is preferred for havingliquid electrolytes. When the electrolyte medium further includes apolymer—such as a gelling polymer—the solvents are advantageouslypresent in an amount comprised between about 50 and 70% by mass, withrespect to 100% by mass of the electrolyte. This range is preferred forhaving gel polymer electrolytes.

The electrolyte medium may further include a component (such as crownethers) that facilitates calcium salt dissociation and/or enhancecalcium salts dissolution.

The electrolyte medium may further include a gelling polymer. This istypically the case of gel polymer electrolytes.

The gelling polymer may be selected from the group consisting ofpolyethylene oxide (PEO), polyvinylidene fluoride (PVDF),polyacrylonitrile (PAN), polymethyl methacrylate (PMMA), poly(vinyl)chloride (PVC), and mixtures thereof.

When the electrolyte medium contains a gelling polymer as defined above,it may further comprise a filler, the filler including:

-   -   a component which can be cross linked and/or thermoset for        example to improve the electrolyte mechanical properties,    -   a plasticizer, for example to improve the electrolyte ionic        conductivity,    -   nanoparticles/nanoceramics, and/or    -   a component (such as crown ethers) that facilitates calcium salt        dissociation and/or enhance calcium salts dissolution.

Nanoparticles/nanoceramics may include Al₂O₃, SiO₂, ZrO₂, MgO, and/orCeO₂ and may have an average particle size equal to or lower than about15 nm (this value can be measured by methods disclosed above). Thiscomponent may be added to increase the electrolyte conductivity.Suitable Al₂O₃ nanoparticles having an average particle size of 5.8 nmare commercially available from Aldrich Research Grade. Suitable SiO₂nanoparticles having an average particle size of 7.0 nm are commerciallyavailable from Aldrich Research Grade,

The filler may be present in an amount of lower than 10% by weight overthe weight of the total gel polymer electrolyte.

The electrolyte presently disclosed was found to lead to the formationof a good quality SEI at the negative electrode when the electrolyte isused in the context of a calcium-based secondary cell. Despite it is notintended to be bound by any theory, it is believed that a good qualitySEI is important for ions proper conductivity thereby improvingcell/battery performances.

In one aspect, the present disclosure discloses a calcium-basedsecondary cell comprising:

-   -   a negative electrode that includes a negative-electrode active        material, said negative-electrode active material being capable        of accepting and releasing calcium ions,    -   a positive electrode that includes a positive-electrode active        material, said positive-electrode active material being capable        of accepting and releasing calcium ions, and    -   an electrolyte arranged between the negative electrode and the        positive electrode, said electrolyte comprising, such as        consisting of calcium ions and an electrolyte medium, wherein        the electrolyte is not solid at standard conditions and wherein        the electrolyte medium includes at least two distinct        non-aqueous solvents.

The negative electrode may be an electrode comprising anegative-electrode active material, said active material includingmetallic calcium or a calcium alloy. Advantageously, the alloy hasformula (I) Ca_(m)B wherein m is 0≦m≦3 and B is a metal or asemi-conductor element.

The negative electrode may consist of a negative electrode activematerial as presently defined.

Preferably, the negative electrode active material includes, such asconsist of, metallic calcium (hereinafter also referred to as “calciummetal”). Preference is due for example to the “theoretical” largecapacity of calcium metal (1.34 Ah/g) and its optimal potential vs.Ca²⁺/Ca.

The negative electrode may be for example a foil of metallic calcium. Inthis case, the metallic calcium may also play the role of currentcollector. A pre-formed, metallic calcium-containing negative electrodecan thus be used during assembly of a calcium-based secondaryelectrochemical cell.

The negative electrode may include, such as consist of a support, suchas a current collector, having a metallic calcium coating asnegative-electrode active material. The coating is obtainable bydepositing metallic calcium on the collector. The coating may be presenton part of the support, only or on the entire support.

The collector may be in the form of a foil, foam or grid.

The collector may comprise, such as consist of copper, aluminum,stainless steel, nickel, gold, platinum, palladium, titanium or carbon.For example, the collector may comprise, such as consist of one or moreof copper, aluminum, stainless steel, nickel, gold, platinum andpalladium. Alternatively, the collector may include, such as consist of,carbon for example type carbon paper. Copper, stainless steel, nickeland carbon, notably carbon and stainless steel, are cost-effectiveoptions. Use of gold or aluminum presents advantages in that thesematerials exhibit the lowest lattice mismatch with calcium. Carbon andaluminum present the advantage to be lighter.

Techniques are known to deposit metallic calcium on a support such as acollector. Electrochemical deposition is a possibility. In situdeposition of metallic calcium on a support previously added during cellassembly is a possibility. In situ deposition may take place while thecell is in use or in charge. In situ deposition is exemplified e.g. inexamples 1 to 5 of the present disclosure. Pulsed Laser Deposition or RFsputtering are other options. In this case, a target of pure calciummetal may be used. This target is commercially available for examplefrom American elements. Nickel foams or grids (on which metallic calciummay be deposited) are also commercially available from Goodfellow. Foamsor grids made of copper or aluminum as well as carbon foams (onepossible supplier of aluminum cupper or carbon foams is ERG-Materials &Aerospace Corporation) or carbon paper (one possible supplier of Carbonpaper is Toray) foils or grids are also commercially available.

Use of a pre-formed, metallic calcium-containing negative electrode andmetallic calcium deposition (e.g. in situ deposition) are not mutuallyexclusive options. If desired, metallic calcium deposition may beperformed on a current collector already made of metallic calcium.

The negative electrode active material may include, such as consists ofa calcium alloy having formula (I) as defined above.

In the negative electrode presently disclosed, the calcium alloy, suchas an alloy of formula (I), may be such that the potential of thenegative electrode is advantageously lower than 2.5V vs. Ca²⁺/Ca, suchas lower than 2V vs. Ca²⁺/Ca, for example lower than 1.8V vs. Ca²⁺/Ca,lower than 1.5V vs. Ca²⁺/Ca. For example, the specific capacity ishigher than 200 mAh/g, such as higher than 300 mAh/g.

Unless otherwise stated, the potentials (in Volt) in the presentdescription and drawings are given versus Ca²⁺/Ca. Potentials aremeasured by a potentiostat versus a Quasi Reference Electrode. Typicallyferrocene or similar internal standard such as cobaltocene is used. Useof ferrocene is known to be suitable for works in non-aqueous media.

In formula (I), B is for example selected from the group consisting oftin (Sn), silicon (Si), germanium (Ge) and mixtures thereof. Forexample, the alloy of formula (I) may be SnCa_(n), SiCa_(p) and/orGeCa_(g) wherein n, p and g are each independently 0≦n, p, g≦3.Variables m, n, p and g are not necessarily integers.

Examples of suitable compounds of formula (I) containing silicon areidentified below through formula, molar mass and theoretical specificcapacity:

Theoretical Molar Specific mass capacity (g/mol) (mAh/g) Si 28.086 3818Ca₃Si 148.32 1084 Ca₂Si 108.24 991 Ca₅Si₃ (or Ca_(5/3)Si) 314.65 852CaSi 68.164 787 Ca₃Si₄ (or Ca_(3/4)Si) 232.58 692 CaSi₂ (or Ca_(0.5)Si)96.250 557

Examples of suitable compounds of formula (I) containing tin areidentified below through formula, molar mass and theoretical specificcapacity:

Theoretical Molar Specific mass capacity (g/mol) (mAh/g) Sn 118.71 903Ca₃Sn 238.94 673 Ca₂Sn 198.71 540 Ca₅Sn₃ (or Ca_(5/3)Sn) 556.52 482Ca₇Sn₆ (or Ca_(7/6)Sn) 992.81 378 CaSn 158.79 338

The negative electrode active material may contain one or more distinctalloys of formula (I).

The negative electrode active material may consist of a calcium alloy asdefined above.

The negative electrode may be a powder composite negative electrode.This electrode is obtainable by processing, such as compressing, amixture (a) including, such as consisting of:

-   -   component (a1) which is the negative electrode active material,        for example a calcium alloy as defined above, and    -   component (a2) which displays electronic conducting properties        and/or electrode volume change constraining properties.

Obtaining mixture (a) may be performed by common techniques. Forexample, mixture (a) can be obtained by simply mixing the variouscomponents for example by means of planetary mills (such as ball millercommercially available from Fritsch).

Component (a1) may be used in an amount comprised between about 50% andabout 100%, preferably between about 65% and about 95%, such as betweenabout 70% and about 90%, for example about 75% with respect to theweight of mixture (a). Component (a2) may be used in an amount comprisedbetween about 0% and about 40%, preferably between about 10% and about30%, for example 25% with respect to the weight of mixture (a).

The properties of component (a2) are thought to be useful when thenegative electrode is in use.

The negative electrode may be a composite film negative electrode. Thiselectrode is obtainable by processing a slurry (b) including, such asconsisting of:

-   -   component (b1) which is the negative electrode active material,        for example a calcium alloy as defined above,    -   component (b2) which displays electronic conducting properties        and/or electrode volume change constraining properties,    -   component (b3) which is a binder,    -   component (b4) which is a solvent.

As evidenced in examples 6 and 7, composite film negative electrodes arepreferred over powder composite negative electrodes.

Component (b1) may be used in an amount comprised between about 50% and90% by weight with respect to the combined weight of components (b1) to(b3), i.e. the solid content of slurry (b). When component (b1) containssilicon, for example a silicon-containing alloy of formula (I), it maybe present in an amount of about 70% by weight with respect to thecombined weight of components (b1) to (b3). When component (b1) containstin, for example a tin-containing alloy of formula (I), it may bepresent in an amount of about 85% by weight with respect to the combinedweight of components (b1) to (b3).

Component (b2) may be used in an amount comprised between about 5% and30% by weight with respect to the combined weight of components (b1) to(b3). When component (b1) contains silicon, for example asilicon-containing alloy of formula (I), component (b2) may be presentin an amount of about 22% by weight with respect to the combined weightof components (b1) to (b3). When component (b1) contains tin, forexample a tin-containing alloy of formula (I), component (b2) may bepresent in an amount of about 7% by weight with respect to the combinedweight of components (b1) to (b3).

Component (b3) may be used in an amount comprised between about 5% and25% by weight with respect to the combined weight of components (b1) to(b3). When component (b1) contains silicon, for example asilicon-containing alloy of formula (I), component (b3) may be presentin an amount of about 8% by weight with respect to the combined weightof components (b1) to (b3). When component (b1) contains tin, forexample a tin-containing alloy of formula (I), component (b3) may bepresent in an amount of about 8% by weight with respect to the combinedweight of components (b1) to (b3).

Component (b4) may be used in any amount suitable to impart a workableviscosity to the slurry. For example, it may be used in an amount ofabout 500% by weight with respect to the combined weight of components(b1) to (b3).

Slurry (b) may further comprise components commonly used in electrodemanufacturing such as component (b5) suitable to impart self-standingproperties to the negative electrode.

Components (a1) and (b1) may be in the form of particles having anaverage particle size falling in the range of 0.01 to 100 microns, suchas in the range of 0.15 to 50 microns. Average particle size may beeither communicated by particles supplier, or measured by e.g. SEM(scanning electron microscopy), TEM (transmission electron microscopy)or laser granulometry techniques.

In the context of slurry (b), component (b2) can typically facilitateslurry preparation and deposition. Components (a2) and (b2) maycomprise, such as consist of particulate carbon. Particulate carbon maybe selected within one or more of carbon black such as ketjen black,acetylene black, channel black, furnace black, lamp black, and thermalblack; graphite, such as natural graphite, e.g., scaly graphite,artificial graphite, and expanded graphite; activated carbon fromcharcoal and coal; carbon fibers obtained by carbonizing syntheticfibers and petroleum pitch-based materials; carbon nanofibers; tubularcarbon, such as carbon nanotubes; and graphene. A suitable conductivecarbon black is Carbon Super P® commercially available from TIMCAL. Themain characteristics of Super P® are their high purity, high structureand their moderate surface area. The high purity is evidenced by the lowash, moisture, sulfur and volatile contents, while their high structureis expressed by oil absorption and electrical conductivity. Super P®conductive carbon black is a carbon black with a high to very high voidvolume originating from the interstices between the carbon blackparticle due to its complex arrangement and porosity, so calledstructure. Such a structure allows retention of a conductive carbonnetwork at low to very low carbon content in the electrode mix. Super P®is a material with no, or nearly no sieve residue on the 325 mesh sieve.

Component (b3) is typically used to ensure the cohesion of the negativeelectrode components. Component (b3) may comprise, such as consist of athermoplastic and/or a thermosetting resin. Component (b3) may beselected from the group consisting of polyethylene, polypropylene,polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF),styrene-butadiene rubber (SBR), carboxymethylcellulose (CMC) or saltsthereof showing various molecular weights and mixtures thereof. Forexample, component (b3) may be a combination of CMC and SBR.

Component (b3) may also be selected from the group consisting oftetrafluoroethylene-hexafluoropropylene copolymers,tetrafluoroethylenehexafluoropropylene copolymers (FEP),tetrafluoroethylene-perfluoroalkyl vinyl ether copolymers (PFA),vinylidene fluoride-hexafluoropropylene copolymers, vinylidene,fluoride-chlorotrifluoroethylene copolymers, ethylenetetrafluoroethylenecopolymers (ETFE resins), polychlorotrifluoroethylene (PCTFE),vinylidene fluoride-pentafluoropropylene copolymers,propylene-tetrafluoroethylene copolymers,ethylene-chlorotrifluoroethylene copolymers (ECTFE), vinylidenefluoride-hexafluoropropylene-tetrafluoroethylene copolymers, vinylidenefluoride-perfluoromethyl vinyl ether-tetrafluoroethylene copolymers,ethylene-acrylic acid copolymers, and mixtures thereof.

Component (b3) may also include a copolymer having sulfonategroup-terminated perfluorovinyl ether groups attached to apoly(tetrafluoroethylene) backbone. An example is copolymerscommercially available under the name Nafion®. For example, thecopolymer may be a dispersion of a copolymer having sulfonategroup-terminated perfluorovinyl ether groups attached to apoly(tetrafluoroethylene) backbone in a mixture of water and 20% byweight of alcohol. This product is commercially available undertrademark LIQUION™ from Ion Power Inc.

Component (b4) is typically used to impart a viscous aspect to slurry(b). Component (b4) may be a solvent selected from the group consistingof acetone, alcohols such as ethanol, cyclic aliphatic hydrocarboncompounds such as cyclohexane, N-methyl-2-pyrrolidone (NMP), propylenecarbonate (PC), N,N-dimethylformamide, tetrahydrofuran (THF), water andmixtures thereof.

An example of component (b5) is plasticizers such as any one or more ofpoly ethylene glycol (PEG), crown-ethers and dibutylphtalate (DBP).

Obtaining slurry (b) may be performed by common techniques. For exampleslurry (b) can be obtained by dispersing solid components (e.g.components (b1) to (b3)) in component (b4) for example by means of ahigh-performance disperser (such as dispersers available from IKA) or anultrasonic disperser (such as dispersers available from Hielscher)or/and by means of a centrifugal mixer (such as commercially availablefrom Thinky). WO2013139370 discloses for example a method formanufacturing a slurry by suspending particulate carbon, a binder andoptionally a catalyst in a solvent.

A composite film negative electrode as defined above (whetherself-standing or supported) may be manufactured by a method comprising astep i) of depositing the negative electrode active material, e.g. inthe form of a slurry (b), on a support.

Depositing may be casting or impregnating, as appropriate depending e.g.on the desired structure of the electrode (self-standing or supported ona current collector and, in this latter case, the type of currentcollectors used).

Casting may be performed by the Doctor Blade method, which allows a finecontrol of the thickness of the active material deposited layer. Forcasting, the support may be in the form of a foil. For casting, thesupport may be made of e.g. copper, aluminum, stainless steel, nickel,gold, platinum, palladium, titanium or carbon if it is a currentcollector or e.g. glass or Teflon for self-standing electrodes.

Impregnating may be performed as disclosed in WO2013139370 (PCTpublication page 16, line 19 onward) wherein a carbon foam support isimpregnated with a slurry containing particulate carbon, a binder, asolvent and optionally a catalyst for the manufacture of a negativeelectrode active material for lithium-air batteries. Typically,impregnating is chosen when the support is a current collector in theform of a foam.

When the negative electrode is designed to be a self-standing one, suchas a self-standing film negative electrode, the method may furthercomprise a step ii-1) of drying the active material deposited on thesupport and a subsequent step iii-1) of removing, for example peelingoff, the support.

Alternatively, the method may further comprise a step ii-2) of dryingthe active material, deposited on the support and a subsequent stepiii-2) of further processing the product obtained in step ii-2). Thisembodiment of the method is suitable to obtain negative electrodes inwhich the support is a current collector as defined above and hence itis part of the final negative electrode. Further processing in stepiii-2) may include a step of heat treating the product of step ii-2).Typically, heat treating is performed at a temperature lower than themelting temperature of the alloy(s) contained in the active material.

Further processing in step iii-2) may include a step of cutting and/orpressing the optionally heat-treated product of step ii-2). Typically,pressing is performed under a pressure between 10⁷ to 10⁹ Pa. Cuttingand pressing may be performed in any order.

Advantageously, the cell may further comprise a temperature controlelement. For example the temperature control element may not bephysically part of (e.g. integral part of) the cell but they may beconfigured to interact. The temperature control element may beconfigured to provide heating functionality and/or coolingfunctionality, e.g. depending on whether cell is used in a context—suchas a fuel engine—wherein a heat source is already present. An elementconfigured to provide at least cooling functionality may be advantageouswhen considering the unavoidable self-heating of the cell when in usedue to the Joule effect. Alternatively, the temperature control elementmay be configured to provide instructions to heating and/or coolingelements present with the cell. Background information on possibletechnical solutions to pre-heat high-voltage battery packs in hybridelectric vehicles up to room temperature (i.e. 25 degrees) can be foundfor example in A. Pesaran et al. “Cooling and Preheating of Batteries inHybrid Electric Vehicles”, The 6^(th) ASME-JSME Thermal EngineeringJoint Conference, Mar. 16-20, 2003, TED-AJ03-633.

The temperature control element is configured to bring and/or maintainthe cell at a temperature between about 30° C. and 150° C., such asbetween about 50° C. and 110° C., which was found to be particularlyeffective for operating the cell presently disclosed. Although it is notintended to be bound by any theory, it is believed that at thistemperature an appropriate conductivity of the SEI can be achieved alsoin those circumstances wherein the SEI is of unsatisfactory quality perse. Accordingly, if the cell is operated in an environment characterizedby low temperatures (such as a device or e vehicle exposed to wintertemperatures), the temperature control element is suitably providedand/or coupled with heating means (for example pre-heating means), suchas a resistance heater and/or a heat pump, so as to bring the cell todesired operating temperature. The temperature control element may alsobe provided with cooling means, for example a fan configured to blowforced air and/or a refrigeration unit configured to operate arefrigeration cycle, so as to maintain the cell within a suitable windowof operating temperatures in case the device of the vehicle is providedwith a heat source such as a fuel engine or the cell, power inverter, orother devices nearby generate heat.

According to some embodiments, the temperature control element mayinclude cooling means only for those applications intended to beconsistently exposed to temperatures above about 30° C. The temperaturecontrol element may be an integral cell thermal management deviceincluding both a heating and a cooling means. The device may be operatedin a controlled manner by e.g. a computer-assisted device (also possiblypart of the temperature control element) so as to reach and maintain anappropriate operating temperature depending on the ambient conditions.

The cell may further comprise a separator. The separator may be a porousfilm or a non-woven fabric. For example, the separator may comprisepolyethylene, polypropylene, cellulose, polyvinylidene fluoride andglass ceramics or mixtures thereof. The separator may contain theelectrolyte. A secondary cell incorporating this embodiment may beobtainable by contacting, e.g. impregnating, the separator with a liquidelectrolyte.

The positive-electrode active material may include any material thatreacts reversibly with calcium preferably at a potential higher than−1.5V expressed versus normal hydrogen electrode (NHE). For example, thepositive-electrode active material may include, such as consist ofoxides of transition metals and/or periodic table Group VI elements,such as vanadium oxides, for example V₂O₅. Crystalline V₂O₅ is anexample. Amorphous V₂O₅-containing compounds, such as compoundscontaining V₂O₅ and crystallization inhibitors are another example.These compounds turned out to be effective in hosting calcium.

The secondary cell may have any form, such as prismatic or cylindricalform.

The secondary cell presently disclosed is calcium based. This means thatthe redox reactions taking place at both electrodes involve calciumions. Put it differently, and given the characteristics of theelectrodes and the electrolyte arranged therebetween, the operationprinciple of the cell involves reaction of the positive and negativeelectrode active materials with calcium ions. This principle isanalogous to that of e.g. conventional lithium metal or lithium-ionsecondary cells but based on calcium instead of lithium. Although it isnot intended to be bound by any theory, it is believed that when thenegative electrode active material includes, such as consists ofmetallic calcium, the chemical hemi-reaction occurring at the negativeelectrode during cell discharge is Ca (metal)→Ca²⁺+2e⁻, while thechemical hemi-reaction occurring during cell charge is Ca²⁺+2e⁻→Ca(metal). Similarly, when the negative electrode active materialincludes, such as consists of a calcium alloy of formula (I) as definedabove, the chemical hemi-reaction occurring at the negative electrodeduring cell discharge is Ca_(m)B (alloy)→Ca²⁺+2m e⁻+B, while thechemical hemi-reaction occurring during cell charge is Ca²⁺+2me⁻+B→Ca_(m)B (alloy).

Advantages associated to the instant calcium-based secondary cell arefor example:

-   -   the energy density of the cell is theoretically higher than that        of commercially available secondary lithium-based cells.        Especially the capacity of the presently disclosed negative        electrodes is higher than that of e.g. graphite-containing        negative electrodes typically used in conventional lithium based        cells, and    -   the cell can effectively be operated at mild temperatures. In        effect, as the electrolyte is non-solid at standard conditions,        there is no need to adopt extremely high operating temperatures        to e.g. keep it in fluid, conducting state (as it would be        necessary had molten calcium salts, such as CaCl₂, be used as        electrolyte). The operating temperatures of the present cell are        acceptable to the vast majority of common applications and        overlap to a large extent with those of conventional        lithium-based cells. This means that the present cell can be        used as power source in conventional information-related        devices, communication devices (e.g. personal computers,        camcorders and cellular phones), in the automobile industry and        in stationary power generating devices.

Advantages of the calcium-based secondary cell presently disclosed overconventional lithium-based cells are inter alia associated with the factthat reaction of each lithium ion involves the transfer of one electronwhile reaction of each calcium ion involves the transfer of twoelectrons. The number of guest calcium ions needed to achieve a certainelectrode capacity is thus half of that of lithium ions, with lowerstructural impact. Alternatively it can be considered that for reactionof the electrode with the same amount of ions, the electrochemicalcapacity is doubled for calcium with respect to lithium. The secondarycell presently disclosed is therefore endowed with an energy densitythat is theoretically higher than that of the commercially availablesecondary lithium-based cells. In particular, the capacity of thenegative electrode presently disclosed is higher than that of e.g.graphite-based negative electrode typically used in lithium-basedsecondary batteries.

In one aspect, the present disclosure discloses a method for operating acalcium-based secondary cell as defined above, said method including thestep of setting cell operating temperature between about 30° C. and 150°C., such as between about 50° C. and 110° C.

Advantageously, the method further includes a step of setting negativeelectrode operating potential window between about −1.5V and 3.0V, suchas −1.5V and 1.8V, versus Ca²⁺/Ca.

The method for operating the cell may further comprise a step ofproviding the cell in a charged state, such as a fully charged state. Incase the negative electrode active material is metallic calcium to bedeposited in situ or the negative electrode active material is an alloyof formula (I) with m equal to 0, the cell is initially in a dischargedstate and thus requires a pre-charging step to be provided in “chargestate” and thus being ready for use. In case the negative electrodeactive material is an alloy of formula (I) with m greater than 0 or ifit already contains metallic calcium (e.g. in case the negativeelectrode is a pre-formed, metallic calcium-containing negativeelectrode), the secondary cell can be directly operated in dischargemode with no pre-charging step needed. This option might be preferredfor certain applications but the alloy shall be prepared prior to cellassembly.

In one aspect, the present disclosure discloses a non-aqueouscalcium-based secondary battery comprising a calcium-based secondarycell as defined above, for example a plurality of calcium-basedsecondary cells wherein at least one is a calcium-based secondary cellas defined above or a plurality of calcium-based secondary cells eachindependently being as defined above. The battery may include one ormore secondary cells as defined above, and a casing. The casing may besurrounded by a temperature control element as defined above, in casethis element is present.

In one aspect, the present disclosure discloses a vehicle, such as amotor vehicle, comprising a non-aqueous calcium-based secondary batteryas defined above.

In one aspect, the present disclosure discloses an electronic device,such as an information-related device or a communication device (forexample a personal computer, camcorder or cellular phone), comprising anon-aqueous calcium-based secondary battery as defined above.

In one aspect, the present disclosure discloses a stationary powergenerating device comprising a non-aqueous calcium-based secondarybattery as defined above.

EXAMPLES

The following experimental examples are illustrative and enable thefunctioning of the invention to be understood. The scope of theinvention is not limited to the specific embodiments describedhereinafter.

Example 1

Calcium metal negative electrode with stainless steel current collectorand 0.3M Ca(ClO₄)₂ in EC_(0.5):PC_(0.5) as electrolyte.

Cyclic voltamperometry (CV) at 1 mV/s was performed in three electrodeSwagelok type tight cells (discussed in D. Guyomard et al., J.Electrochem. Soc. (1992), vol. 139, p. 937) with pieces of calcium metal(provided by Aldrich) as counter and reference electrodes. The workingelectrode current collector was stainless steel. Two sheets of WhattmanGF/d borosilicate glass fiber were used as a separator, soaked with theelectrolyte (ca. 0.5 cm³ of 0.3M Ca(ClO₄)₂ in EC_(0.5):PC_(0.5)). CVswere performed at various temperatures (i.e. 25, 50 and 75° C.) andpotential was swept between −1 and 2 V vs. Ca²⁺/Ca (see FIG. 2).Significant oxidation and reduction currents were measured fortemperatures above 25° C. (room temperature in FIG. 2). Indeed, anoxidation peak was observed for potential slightly higher than 0 V vs.Ca²⁺/Ca which could be attributed to calcium stripping and the reductionprocess occurring at potentials lower than ca. −0.5 V vs. Ca²⁺/Ca couldbe attributed to calcium plating.

Example 2

Calcium metal negative electrode with stainless steel current collectorand 0.3M Ca(BF₄)₂ in EC_(0.5):PC_(0.5) as electrolyte.

In this example 2, the deposition was performed at 75° C. in apotentiostatic mode (contrary to the CV technique of example 1). Apicture of a stainless steel plunger electrode after calcium depositionis displayed in FIG. 3 and a grey metallic like deposit is visible.

Example 3

Calcium metal negative electrode with copper current collector and 0.3MCa(BF₄)₂ in EC_(0.45):PC_(0.45):DMC_(0.1) as electrolyte.

Calcium electroplating was performed at 75° C. for 200 h bypotentiostatic deposition at −1 and −1.5 V vs. Ca²⁺/Ca using twoelectrode Swagelok type tight cells with pieces of calcium metal ascounter and reference electrode. 20 micron (μm) thick copper disks orstainless steel plungers were used as the working electrodes (i.e.calcium plating substrate). Two sheets of Whattman GF/d borosilicateglass fiber were used as a separator, soaked with the electrolyte (ca.0.5 cm³ of 0.3M Ca(BF₄)₂ in EC_(0.45):PC_(0.45):DMC_(0.1)). Scanningelectron microscopy images of copper substrate prior to calciumdeposition are shown in FIG. 4a . After 80 h deposition at −1 V vs.Ca²⁺/Ca a deposit composed of micron size agglomerates can beappreciated (FIG. 4b ). The deposit is very thin and the coppersubstrate can still be observed. For longer deposition time (i.e. 200 h)and at −1.5 V vs. Ca²⁺/Ca a much thicker deposit is achieved (FIG. 4c )and from EDS analysis the main elements detected are calcium, fluorineand oxygen, the copper EDS peaks being barely detected (<5% at., FIG.5), which indicates that the calcium deposit is at least micron thick.Thick deposits were scratched from the surface of the working electrodeand filled inside capillaries that were further sealed under Argonatmosphere. X-ray diffraction was performed using synchrotron radiation(ALBA) and aside from substrate contaminations all peaks (FIG. 6) an beattributed to either calcium metal or CaF₂, the latter being a componentof the solid electrolyte interphase due to electrolyte decomposition.

Example 4

Calcium metal negative electrode with various current collectors and0.3M Ca(BF₄)₂ in EC_(0.45):PC_(0.45):DMC_(0.1) as electrolyte.

The same electrochemical set up was used as in example 3, except that wetested here the effect of various current collectors (FIG. 7).

Example 5

Calcium metal negative electrode with current collector composed of golddeposited onto the stainless steel and xM Ca(BF₄)₂ inEC_(0.45):PC_(0.45):DMC_(0.1) as electrolyte (x=0.3; 0.65; 1).

The same electrochemical set up was used as in example 3, except that wetested here the effect of various salt concentrations (FIG. 8).

Example 6

Composite film containing tin as negative electrode with copper currentcollector and 0.3M Ca(BF₄)₂ in EC_(0.45):PC_(0.45):DMC_(0.1) aselectrolyte.

Slurries were prepared by mixing 85 wt. % of active material (i.e. tinparticles having an average particle size of 150 nm, provided byAldrich), 8 wt. % of PVDF as a binder and 7 wt. % of C_(sp) as carbonadditive in NMP. Mixing of the slurries was performed by magneticstirring during 3 h, the vial containing the slurry being placed in anultrasonic bath for 10 minutes every 1 h. Composite electrodes wereprepared by depositing the slurry on a 20 micron thick copper foil witha 250 micron Doctor-Blade and further dried at 120° C. under vacuum.Once dried, 0.8 cm² disk electrodes were cut and pressed at 7.8×10⁸ Pa.The resulting tin-containing composite film electrodes were then testedas anode in Swagelok type cells with a dendritic piece of calcium metalas counter and reference electrodes. Two sheets of Whattman GF/dborosilicate glass fiber were used as a separator, soaked with theelectrolyte (ca. 0.5 cm³ of 0.3M Ca(BF₄)₂ inEC_(0.45):PC_(0.45):DMC_(0.1)). Temperature was set at 75° C. duringelectrochemical tests using Potentiodynamic Intermittent TitrationTechnique (PITT) with potential steps of 5 mV. The potential was steppedto the next value when the current dropped below 2 μA (corresponding toa rate of C/400), meaning one mol of Ca²⁺ per mol of tin inserted in 400hours. Upon the first reduction, specific capacity as high as 1770 mAh/gis obtained (FIG. 9). Upon the first oxidation a capacity of ca. 1027mAh/g was recorded indicating the reversibility to some extent of theredox process occurring upon reduction. Scanning electron microscopyimages were taken on powder samples before and after full reduction downto −0.5 V vs Ca²⁺/Ca (FIG. 10) and the particle size distribution (FIG.11) was evaluated by randomly measuring the diameter of 100 particlesusing “ImageJ’ software. A significant tin particle sizes increase wascalculated after reduction with the mean diameter of the tin particlesevolving from ca. 0.5 μm for the pristine powder to ca. 1.1 μm afterfull reduction as expected for the formation of SnCa_(n) alloys.

Example 7

Composite powder containing tin as negative electrode with stainlesssteel plunger as current collector and 0.3M Ca(BF₄)₂ inEC_(0.45):PC_(0.45):DMC_(0.1) as electrolyte

Powders were prepared by mixing 75 wt. % of active material (i.e. tinparticles having an average particle size of 150 nm, provided byAldrich) and 25 wt. % of C_(sp) as carbon additive. Mixing of thepowders was performed by hand milling using an agate mortar during 15min. Composite powder electrodes were tested as anode in Swagelok typecells with a dendritic piece of calcium metal as counter and referenceelectrodes. Two sheets of Whattman GF/d borosilicate glass fiber wereused as a separator, soaked with the electrolyte (cs. 0.5 cm³ of 0.3MCa(BF₄)₂ in EC_(0.45):PC_(0.45):DMC_(0.1)). Temperature was set at 75°C. during electrochemical tests using Potentiodynamic IntermittentTitration Technique (PITT) with potential steps of 5 mV. The potentialwas stepped to the next value when the current dropped below 2 μA(corresponding to a rate of C/400), meaning one mole of Ca²⁺ per mole oftin inserted in 400 hours. Scanning electron microscopy images weretaken on powder samples before and after full reduction down to −0.5 Vvs Ca²⁺/Ca (FIG. 10) and the particle size distribution (FIG. 11) wasevaluated by randomly measuring the diameter of 100 particles using“ImageJ” software. A significant tin particle sizes increase wascalculated after reduction with the mean diameter of the tin particlesevolving from ca. 0.5 μm for the pristine powder to ca. 1.1 μm afterfull reduction as expected for the formation of SnCa_(n) alloys.

Example 8

Composite film containing silicon as negative electrode with coppercurrent collector and 0.3M Ca(BF₄)₂ in EC_(0.45):PC_(0.45):DMC_(0.1)

Slurries were prepared by mixing 70 wt. % of the active material (i.e.silicon particles having average particle size of 325 mesh, provided byAldrich), 8 wt. % of PVDF as a binder and 22 wt. % of C_(sp) as carbonadditive in NMP. Mixing of the slurries was performed by magneticstirring during 3 h, the vial containing the slurry being placed in anultrasonic bath for 10 minutes every 1 h. Composite electrodes wereprepared by depositing the slurry on a 20 micron thick copper foil witha 250 micron Doctor-Blade and further dried at 120° C. under vacuum.Once dried, 0.8 cm² disk electrodes were cut and pressed at 7.8×10⁸ Pa.The resulting silicon-containing composite film electrodes were testedas anode in Swagelok type cells with a dendritic piece of calcium metalas counter and reference electrodes. Two sheets of Whattman GF/dborosilicate glass fiber were used as a separator, soaked with theelectrolyte (ca. 0.5 cm³ of 0.3M Ca(BF₄)₂ inEC_(0.45):PC_(0.45):DMC_(0.1)). Temperature was set at 75° C. duringelectrochemical tests using Potentiodynamic Intermittent TitrationTechnique (PITT) with potential steps of 5 mV. The potential was steppedto the next value when the current dropped below 5 μA (corresponding toa rate of C/400), meaning one mol of Ca²⁺ per mol of tin inserted in 400hours. Upon the first reduction specific capacity as high as 1589 mAh/g(FIG. 12) was obtained related to a pseudo plateau centered at ca. 0.6 Vvs. Ca²⁺/Ca.

Example 9

Calcium metal negative electrode with stainless steel current collectorand 0.3M Ca(BF₄)₂ in EC_(0.5):PC_(0.5) as electrolyte.

In this example 9, cyclic voltammetry (CV) at 0.5 mV/sec was performedat 100° C. for 20 cycles in three electrode Swagelok type cell withpieces of calcium metal (provided by Aldrich) as counter and referenceelectrodes. The working electrode current collector was stainless steel.Two sheets of Whattman GF/d borosilicate glass fiber were used as aseparator, soaked with the electrolyte (ca. 0.5 cm³ of 0.3M Ca(BF₄)₂ inEC_(0.5):PC_(0.5)). CVs were performed at 100° C. and potential wasswept between −1.5 and 2 V vs. Ca²⁺/Ca (see FIG. 13). Significantoxidation and reduction currents were measured. Onset potentials forplating and stripping of calcium are respectively, ca. −0.68 V and ca.−0.5 V vs. Ca2+/Ca

1.-17. (canceled)
 18. An electrolyte comprising calcium ions and anelectrolyte medium, wherein the electrolyte is not solid and has aviscosity of lower than 10 cP at a temperature of about 20° C. and apressure of 1 atm and wherein the electrolyte medium includes at leasttwo distinct non-aqueous solvents.
 19. The electrolyte according toclaim 18, wherein each of the at least two distinct non-aqueous solventsis independently selected from the group consisting of cycliccarbonates, linear carbonates, cyclic esters, cyclic ethers, linearethers and mixtures thereof.
 20. The electrolyte according to claim 19,wherein the medium includes ethylene carbonate (EC).
 21. The electrolyteaccording to claim 20, wherein the medium includes ethylene carbonate(EC) and propylene carbonate (PC).
 22. The electrolyte according toclaim 21, wherein the medium includes a combination of formulaEC_(h):PC_(1-h) wherein the ratio is expressed as volume:volume and h is0≦h≦1.
 23. The electrolyte according to claim 21, wherein the mediumincludes a combination of ethylene carbonate (EC), propylene carbonate(PC) and dimethyl carbonate (DMC), having formula EC_(x):PC_(y):DMC_(z)wherein the ratio is expressed as volume:volume and 0≦x,y,z≦1 andx+y+z=1.
 24. A calcium-based secondary cell comprising: a negativeelectrode that includes a negative-electrode active material, saidnegative-electrode active material being capable of accepting andreleasing calcium ions, a positive electrode that includes apositive-electrode active material, said positive-electrode activematerial being capable of accepting and releasing calcium ions, anelectrolyte arranged between the negative electrode and the positiveelectrode, said electrolyte being as defined in claim
 18. 25. The cellaccording to claim 24, further comprising a temperature control element.26. The cell according to claim 24, wherein the negative electrodecomprises a negative-electrode active material, said active materialcomprising metallic calcium or a calcium alloy.
 27. The cell accordingto claim 26, wherein the negative electrode active material includesmetallic calcium.
 28. The cell according to claim 26, wherein thenegative-electrode active material comprises a calcium alloy which issuch that the potential of the negative electrode is lower than about2.5V vs. Ca²⁺/Ca and has a specific capacity which is higher than about200 mAh/g.
 29. The cell according to claim 26, wherein the alloy hasformula (I) Ca_(m)B wherein m is 0≦m≦3 and B is a metal or asemi-conductor element.
 30. The cell according to claim 18, wherein thenegative electrode is a composite film negative electrode.
 31. A methodfor operating a calcium-based secondary cell according to claim 24, saidmethod including the step of setting cell operating temperature betweenabout 30° C. and 150° C.
 32. A non-aqueous calcium-based secondarybattery comprising a calcium-based secondary cell according to claim 24.33. A vehicle, an electronic device or a stationary power generatingdevice comprising a non-aqueous calcium-based secondary batteryaccording to claim 32.